Aquatic Xylaria: an exotic fungus introduced into the United States on aquarium decorative wood

This study examined fungi with Xylaria-like morphology on submerged decorative wood in freshwater aquariums in Minnesota and Colorado. The wood was sold in retail stores in the United States but originated from Asia. The submerged wood had black stromatic melanized structures with white tips that grew out from the wood. As colonization progressed, the fungus produced more melanized structures along the entire length of the wood and moved to new wood placed in the aquariums. Cut segments from the fungal structures and from the colonized wood were cultured in malt extract agar supplemented with antibiotics. Pure cultures obtained were used for DNA extraction, and polymerase chain reaction (PCR) amplification was performed using ITS1F-ITS4. Sequences were compared against the National Center for Biotechnology Information (NCBI) nucleotide database using BLASTn. Isolates from the fungal structures and wood obtained from the Minnesota and Colorado aquariums were all found to be Xylaria apoda, a fungus not previously reported from the United States. Phylogenetic analysis shows that the aquarium isolates of Xylaria cluster within a well-supported clade of Xylaria apoda. These Xylaria had grown in a completely aquatic habitat, producing fruiting body–like structures for several years in the freshwater aquariums. Asexual spores were not observed but may have been disseminated into the water as they are formed. Asci and ascospores were also not observed. Xylaria apoda has been reported only from Asia, and it is usually found in terrestrial habitats. This report adds to our knowledge of Xylaria that can grow in a completely underwater environment and focuses attention on an avenue for exotic fungi to be brought into new countries where they are not native. These results also contribute to the growing body of evidence that X. apoda is an ecologically versatile species, capable of thriving in diverse environments, including artificial habitats such as freshwater aquariums.